I too deeply regret the upgrade to Monterey. I heavily rely on Sidecar when working remotely for that extra screen! Everything has worked great for a couple of years until yesterday. I am on 12.4 and 15.5, 16" MBP and iPad Pro.
I figured out Sidecar really lives in Control Center now--fine. But the AirPlay button on the TouchBar is still there but doesn't do anything. Nice attention to detail.
What has killed this for me are the following:
- The Dock and Menu Bar are missing from the iPad. They were both there until the upgrade: I don't think the intention is to mouse all the way back to the other screen just to hit a menu while Mail is on the iPad screen, folks. They were both there too until I clicked and held the green button in Mail and sent Mail to the iPad screen. Now, no Dock or Menu Bar on the extended screen. Unusable now.
- Ever since day one of owning the iPad Pro, I've used it as a second monitor. The only trouble I've had were by accident, usually killing the battery. But now, ouch. Even using the 0.3m ThunderBolt cable that I've always used, the iPad will freeze up: cursor sticks to the screen even though is works fine on Mac, or it'll just go blurry/grey. A hard reset of the iPad is all that fixes it.
- Wireless Sidecar seems to work, but exhibits the same problems. On the topic of WiFi, I get the feeling Sidecar still uses WiFi even though the iPad is connected via the TB cord. That doesn't make sense. How can I know for sure, other than putting iPad into Airplane Mode?
Man, I've been suckered into upgrades before, and promised never to do it again on my money maker machines. Both of my servers are running steady on El Cap! And since current versions of Server are just toys, they're staying on El Cap! I thought Monterey had been out long enough to smooth things out: major things too! Boy was I wrong. I was lazy too, and didn't check out the forums first!
Any advice would be welcome.